The invention relates to a hydraulic steering system and method having a steering linkage and an actuator actuating the steering linkage. Pressurized hydraulic fluid can be fed to the actuator and removed through a return pipe.
A hydraulic steering system may be constructed as a rack-and-pinion hydro-steering system and may include a pump by which hydraulic fluid can be delivered under pressure to an actuator of the rack-and-pinion hydro-steering system. In this manner, the steering force that has to be applied by the driver of a vehicle can be reduced. By way of an inflow pipe, the pump is connected with the rack-and-pinion hydro-steering system, and a return pipe is also provided by which hydraulic fluid can be delivered back into a storage tank from the rack-and-pinion hydro-steering system.
In the case of such hydraulic open-center steering systems, axle vibrations, which arise when driving over uneven ground in the steered condition, result in excessively fast movements of the rack of a rack-and-pinion hydro-steering system. These movements as well as the load fluctuations connected therewith cause high volume flow rate peaks in the steering system.
In contrast, the pump delivers a constant flow volume into the steering system, so that volume flow rate valleys occur subsequent to the volume flow rate peaks. In the return pipe of the steering system, these volume flow rate valleys result in considerable pressure drops below the ambient pressure. Under certain circumstances, this may have the result that the volume flow breaks off and, because of the mass characteristics of the fluid, a “tensile stress” occurs in the flowing fluid column. Thus, cavitation bubbles, in which hydraulic fluid evaporates, may occur in the return pipe. These cavitation bubbles collapse at a very high speed and, in the process, generate high-frequency pressure peaks. These pressure peaks lead to a structure-borne noise excitation of the components conducting the hydraulic fluid, which is clearly audible in the vehicle and represents an acoustic quality problem.
Depending on the natural frequency of the entire axle kinematics of the steering system, this acoustic phenomenon occurs several times successively with a frequency of approximately 10 to 20 Hz. The individual events therefore follow one another at very short intervals and thereby generate a rattling noise. The driver of the vehicle would feel that such a noise is particularly disturbing.
German patent document DE 197 29 777 C2 describes damping valves for a power steering system constructed as described above. Here, one damping valve, respectively, is situated at the inflow and outflow of the actuator, between the actuator and the steering valve. The two damping valves act reciprocally, each in one flow direction.
In the case of other known steering systems, the noise problem is combated by a damming-up of the return pipe by way of an orifice plate which is arranged in the center or at the end of the return pipe. By way of the orifice plate, an increased pressure is obtained in the return pipe, by which the formation of cavitations is prevented. However, at the same time, an undesirable loss of pressure is caused by which the power consumption of the steering system is increased. The fuel consumption fraction for the steering system will therefore be rising. Furthermore, a higher cooling capacity will be required in extreme driving situations with a high thermal load (for example, during a high-speed drive or an uphill drive).
It is an object of the invention to provide a hydraulic steering system which, while its construction is simple, permits such an exact proportioning of the time response that the mentioned acoustic problems will not occur. In addition, only low energy expenditures should be required for the steering system.
According to the invention, a hydraulic steering system is provided having a steering linkage and an actuator actuating the steering linkage, to which actuator pressurized hydraulic fluid can be fed and from which the hydraulic fluid can be removed through a return pipe. A device is installed in the return pipe which counteracts volume flow pulsations by damming up the hydraulic fluid return flow. This device is constructed such that it dams up the hydraulic fluid only in the event of the occurrence of a volume flow pulsation.
When a flow resistance, which can be varied as a function of pressure, is applied to the discharged hydraulic fluid flow in the return pipe from the actuator to a storage tank, particularly in front of the storage tank and behind the steering valve, this has the advantage that a permanent loss of pressure is avoided while the acoustics are good. The flow resistance is constructed, for example, as a damping valve, which dams up the return flow only in the event of the occurrence of a volume flow pulsation, and thus has a damming effect only as required. The fuel consumption fraction of the steering is reduced.
When a volume flow rate peak in the hydraulic system occurs in the case of a steering system improved in this manner, which volume flow rate peak clearly exceeds the nominal volume flow rate, the return flow will be dammed up according to the invention. As a result, the pressure loss in the return pipe will rise and a damping effect will occur according to the requirement without the occurrence of a continuous damming-up of hydraulic fluid and energy consumption connected therewith.
According to a further embodiment of the invention, the device counteracting volume flow pulsations preferably consists of a hydraulic fluid storage volume, which can be enlarged against a restoring force and which can be filled and emptied by way of a resistance in the valve slide, which acts as a throttle. A valve chamber having a main sectional flow area and a secondary sectional flow area is installed into the return pipe, which valve chamber displaceably carries the valve slide for closing the main sectional flow area. The valve slide has two piston surfaces which respond to the pressure in the hydraulic fluid and are situated opposite one another in the displacement direction, one of the two piston surfaces having a larger effective surface in the storage volume and another having a smaller effective surface in the sectional flow area of the return flow. The piston surfaces are selected such that, on the one hand, when the hydraulic fluid return flow is free of pulsations, the valve slide is held in the valve chamber with an open main sectional flow area and, on the other hand, as a result of the occurrence of volume flow pulsations, is displaced into a position closing the main sectional flow area. Subsequently, the throttle, the secondary sectional flow area and the storage volume loaded by the restoring force together with the valve slide form a passively switched time delay element, which permits the automatic axial displacement of the valve slide into the opening position for the main sectional flow area only when the volume flow pulsations have been damped and/or have subsided.
Based on the mechanically hydraulic method of operation, the device can advantageously be constructed with fewer components and can be integrated in the return pipe without any significant expenditures. Moreover, it can virtually be installed as one component, and no additional components have to be mounted.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.